diff --git a/.changeset/fn-select-groupby-error.md b/.changeset/fn-select-groupby-error.md
new file mode 100644
index 000000000..cfc87883d
--- /dev/null
+++ b/.changeset/fn-select-groupby-error.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/db': patch
+---
+
+fix(db): throw error when fn.select() is used with groupBy()
diff --git a/packages/db/src/errors.ts b/packages/db/src/errors.ts
index dc1c7b900..f62278c56 100644
--- a/packages/db/src/errors.ts
+++ b/packages/db/src/errors.ts
@@ -433,6 +433,17 @@ export class DistinctRequiresSelectError extends QueryCompilationError {
   }
 }
 
+export class FnSelectWithGroupByError extends QueryCompilationError {
+  constructor() {
+    super(
+      `fn.select() cannot be used with groupBy(). ` +
+        `groupBy requires the compiler to statically analyze aggregate functions (count, sum, max, etc.) in the SELECT clause, ` +
+        `which is not possible with fn.select() since it is an opaque function. ` +
+        `Use .select() instead of .fn.select() when combining with groupBy().`,
+    )
+  }
+}
+
 export class HavingRequiresGroupByError extends QueryCompilationError {
   constructor() {
     super(`HAVING clause requires GROUP BY clause`)
diff --git a/packages/db/src/query/compiler/index.ts b/packages/db/src/query/compiler/index.ts
index d0d7469e2..885a7eaa6 100644
--- a/packages/db/src/query/compiler/index.ts
+++ b/packages/db/src/query/compiler/index.ts
@@ -4,6 +4,7 @@ import {
   CollectionInputNotFoundError,
   DistinctRequiresSelectError,
   DuplicateAliasInSubqueryError,
+  FnSelectWithGroupByError,
   HavingRequiresGroupByError,
   LimitOffsetRequireOrderByError,
   UnsupportedFromTypeError,
@@ -218,6 +219,10 @@ export function compileQuery(
     throw new DistinctRequiresSelectError()
   }
 
+  if (query.fnSelect && query.groupBy && query.groupBy.length > 0) {
+    throw new FnSelectWithGroupByError()
+  }
+
   // Process the SELECT clause early - always create $selected
   // This eliminates duplication and allows for DISTINCT implementation
   if (query.fnSelect) {
diff --git a/packages/db/tests/query/group-by.test.ts b/packages/db/tests/query/group-by.test.ts
index 0875ec141..3187736a0 100644
--- a/packages/db/tests/query/group-by.test.ts
+++ b/packages/db/tests/query/group-by.test.ts
@@ -2167,6 +2167,31 @@ function createGroupByTests(autoIndex: `off` | `eager`): void {
         expect(result?.totalAmount).toBe(700)
       })
     })
+
+    describe(`fn.select with groupBy throws error`, () => {
+      let ordersCollection: ReturnType<typeof createOrdersCollection>
+
+      beforeEach(() => {
+        ordersCollection = createOrdersCollection(autoIndex)
+      })
+
+      test(`fn.select with groupBy should throw FnSelectWithGroupByError`, () => {
+        expect(() =>
+          createLiveQueryCollection({
+            startSync: true,
+            query: (q) =>
+              q
+                .from({ orders: ordersCollection })
+                .groupBy(({ orders }) => orders.customer_id)
+                .fn.select((row) => ({
+                  customerId: row.orders.customer_id,
+                  totalAmount: sum(row.orders.amount),
+                  orderCount: count(row.orders.id),
+                })),
+          }),
+        ).toThrow(`fn.select() cannot be used with groupBy()`)
+      })
+    })
   })
 }